Without limiting the scope of the present invention, its background will be described with reference to perforating a subterranean formation with a perforating gun assembly, as an example.
After drilling a section of a subterranean wellbore that traverses a formation, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path for producing fluids from the producing intervals to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic openings or perforations must be made through the casing string, the cement and a short distance into the formation.
Typically, these perforations are created by detonating a series of shaped charges that are disposed within the casing string and are positioned adjacent to the formation. Specifically, one or more charge carriers are loaded with shaped charges that are connected with a detonator via a detonating cord. The charge carriers are then connected within a tool string that is lowered into the cased wellbore at the end of a tubing string, wireline, slick line, electric line, coil tubing or other conveyance. Once the charge carriers are properly positioned in the wellbore such that the shaped charges are adjacent to the interval to be perforated, the shaped charges may be fired. Upon detonation, each shaped charge generates a high-pressure stream of metallic particles in the form of a jet that penetrates through the casing, the cement and into the formation.
The goal of the perforation process is to create openings through the casing to form a path for the effective communication of fluids between the reservoir and the wellbore. It has been found, however, that a variety of factors associated with the perforating process can significantly influence the productivity of the well. For example, during the drilling phase of well construction, drilling mud particles build up a filter cake on the side of the wellbore. While the filter cake prevents additional leaching of drilling mud into the reservoir, this filtrate may impair production from the reservoir. Accordingly, effective perforations must not only be formed through the casing and cement, but also through this filter cake and into virgin rock.
As another example, the pressure condition within the wellbore during the perforation process has a significant impact on the efficiency of the perforations. Specifically, perforating may be performed in an overbalanced or underbalanced pressure regime. Perforating overbalanced involves creating the opening through the casing under conditions in which the hydrostatic pressure inside the casing is greater than the reservoir pressure. Overbalanced perforating has the tendency to allow the wellbore fluid to flow into the reservoir formation. Perforating underbalanced involves creating the opening through the casing under conditions in which the hydrostatic pressure inside the casing is less than the reservoir pressure. Underbalanced perforating has the tendency to allow the reservoir fluid to flow into the wellbore. It is generally preferable to perform underbalanced perforating as the influx of reservoir fluid into the wellbore tends to clean up the perforation tunnels and increase the depth of the clear tunnel of the perforation.
It has been found, however, that even when perforating is performed underbalanced, the effective diameter of the perforation tunnels is small as the jet of metallic particles that creates the perforation tunnels is highly concentrated. Due to the small diameter of the perforation tunnels, the volume of the perforation tunnels is also small. In addition, it has been found that even when perforating is performed underbalanced, the surface of the perforation tunnels has reduced permeability compared to the virgin rock.
Therefore a need has arisen for a perforating gun assembly having shaped charges that produce jets that are capable of penetrating through the casing, the cement, the filter cake and into the virgin rock of the reservoir formation. A need has also arisen for such a perforating gun assembly that is not limited to creating small volume perforation tunnels behind the casing. Further, a need has arisen for such a perforating gun assembly that is not limited to creating perforation tunnels having a surface with reduced permeability compared to the virgin rock.